Deposition of a metal film in semiconductor processing often requires desorb and preclean steps prior to the deposition of the metal film. The desorb and preclean steps assure good adhesion of the metal film to a substrate and also provides better contact resistance between a substrate metal and the metal film.
In the prior art, the desorb step, the preclean step, and the deposition of the metal film take place within a cluster tool so that the substrate is not exposed to atmosphere between the desorb or preclean steps and the deposition of the metal film.
The desorb step of the prior art heats the substrate under vacuum in order to degas the substrate. By heating the substrate under vacuum, material adsorbed to the surface of the substrate or absorbed within the substrate are removed from the substrate. Typical process conditions include vacuum of 10−3 Torr or higher vacuum, and temperature within the range of 200 and 400° C. Generally, higher temperatures are employed in order to minimize process times, which for the higher temperatures are generally within the range of 30 and 60 s.
The preclean step of the prior art exposes the substrate to ion bombardment in what is often referred to as a sputter-etch preclean. In the sputter-etch preclean, argon ions, hydrogen ions, helium ions, or some combination thereof, and electrons form a plasma, which bombards a surface of the substrate to sputter away a thin layer of material. Typically, in the semiconductor processing, an etching step precedes the deposition of the metal film. The etching step forms trenches and via holes in the substrate to an underlying metal layer. After the etching step, an oxide forms on an exposed surface of the underlying metal layer at the via holes due to exposure of the substrate to atmosphere. The sputter-etch preclean attempts to etch away the oxide and any remaining residue after ashing and wet cleaning. The sputter-etch preclean typically requires a vacuum of 10−3 Torr or higher vacuum.
A particular metal deposition process of the prior art deposits a barrier metal layer and a copper seed layer onto a semiconductor substrate forming contacts with an underlying copper layer at the via holes. In the etching step, the via holes are formed through silicon dioxide and silicon nitride layers to the underlying copper layer. In another etching step of the prior art, the via holes are formed in low-k dielectric materials such as a polymer-based materials and fluorine or carbon containing oxides. After the etching step and before the deposition of the metal film, a plasma ashing step and a wet cleaning step substantially remove photoresist, photoresist residue, and etch residue leaving material adsorbed to the surface of the substrate and leaving a thin copper oxide layer on the underlying copper layer at the via holes. The desorb step of the prior art removes the material adsorbed to the surface of the substrate. The sputter-etch preclean removes the copper oxide layer within the via holes and removes an exposed layer of the substrate surrounding the via holes. The barrier metal is then deposited followed by the copper seed layer. Subsequently, an electroplating step deposits an additional copper layer on the copper seed layer.
The cluster tool of the prior art includes a handoff station, a front transfer module, a back transfer module, a degas module, a sputter-etch module, and a metal deposition module. The front transfer module includes a first robot. The back transfer module includes a second robot. The handoff station is coupled to the front transfer module by a first valve or loadlock. The degas module and the sputter-etch module are coupled to the front transfer module. The back transfer module is coupled to the front transfer module by a second valve or loadlock. The metal deposition module is coupled to the back transfer module. Often, the cluster tool includes two degas modules, two sputter-etch modules, and two or more metal deposition modules. In operation, the front and back transfer modules operate at vacuum. A second cluster tool for performing the desorb, preclean and metal deposition of the prior art includes a single transfer module, the degas module, the sputter-etch module, and the metal deposition module, where the degas module, the sputter-etch module, and the metal deposition module are coupled to the single transfer module.
Operation of the cluster tool begins with the first robot transferring a substrate from the handoff station to the degas module, where the desorb step takes place. The first robot then transfers the substrate to the sputter-etch module where the sputter-etch preclean takes place. The first robot then transfers the substrate to the second robot, which places the substrate in the metal deposition module. After the deposition of the metal film, the second robot then returns the substrate to the first robot, which returns the substrate to the handoff station.
Because the desorb step of the prior art operates at elevated temperatures, there is potential for temperature induced damage of the substrate. This concern is especially apt for polymer materials because future integrated circuits may employ the polymer materials as insulators due to their low dielectric constant properties.
In the preclean step, the plasma can cause plasma damage of the surface of the substrate. Also, it is well known that the sputter-etch preclean causes corner clipping at edges of the trenches and at edges of the via holes creating facets. The corner clipping is especially detrimental to smaller dimension integrated circuits since the corner clipping reduces separation of adjacent lines leading to unacceptable electrical interference between the adjacent lines. Not only does the sputter-etch preclean cause physical damage of integrated circuits, it could also causes electrical damage.
Further, sputtering of the underlying metal layer in the trenches and the via holes can cause barreling as well as causing deposition of sputtered material on sidewalls of the trenches and the via holes. For example, sputter-etch preclean of the copper oxide layer within the via holes causes copper and copper oxide to deposit on the sidewalls of the via holes. Moreover, the sputter-etch preclean is inappropriate for precleaning polymer based materials due to expected damage caused by the physical bombardment in the sputter-etch preclean. Also, even if the sputter-etch preclean can be used, the hydrogen ions cannot be used when the polymer materials are exposed since the hydrogen will hydrate the polymer-based materials. Additionally, the sputter-etch preclean becomes less effective as an aspect ratio (depth divided by width) of the trenches and of the via holes increases.
Both the degas module and the sputter-etch module require high vacuum pumps and associated vacuum plumbing, which increases purchase and maintenance costs of the cluster tool. The sputter-etch module further increases the purchase and maintenance costs of the cluster tool because it relatively complex and requires frequent maintenance to ensure that it does not become a source of particulate contamination.
What is needed is a desorb method compatible with metal deposition that does not require excessive temperatures.
What is needed is a preclean method compatible with metal deposition that does not use a plasma.
What is needed is a desorb method compatible with low-k materials such as polymer materials, and fluorine or carbon containing oxides.
What is needed is a preclean method compatible with low-k materials such as polymer materials, and fluorine or carbon containing oxides.
What is needed is a desorb method compatible with metal deposition which is less expensive.
What is needed is a preclean method compatible with metal deposition which is less expensive.